Sensing xenobiotic compounds: Lessons from bacteria that face pollutants in the environment
Víctor de Lorenzo, Rafael Silva-Rocha, Guillermo Carbajosa, Teca C. Galvão and Ildefonso Cases
from: Sensory Mechanisms in Bacteria: Molecular Aspects of Signal Recognition (Edited by: Stephen Spiro and Ray Dixon). Caister Academic Press, U.K. (2010)
Bacteria that inhabit sites with a history of pollution by chemical waste possess an astonishing ability of evolving new pathways for catabolism of otherwise recalcitrant and/or xenobiotic compounds. The emergence of such pathways is often accompanied by the appearance of transcriptional regulatory circuits that adjust the levels of biodegradative activity to the available concentrations of cognate substrate(s). In addition, such circuits compute substrate levels along with their toxicity. In this way, the corresponding sensor systems implement a distribution of cell resources between functions for enduring stress and for metabolization of the target chemical. More than 90 transcription factors belonging to 10 different protein families are known to this day to regulate expression of biodegradative genes and operons for catabolism of persistent and xenobiotic molecules. In other cases, a number of regulatory proteins control expression of extrusion pumps for toxic chemicals or an excess of metabolic intermediates thereof. Experimental evolution studies made with a diversity of prokaryotic regulators that respond to such compounds (XylR, XylS, NahR, TetR and others) have revealed some mechanisms by which non-natural small molecules may become inducers upon binding these proteins. In some studied cases, such factors regress into functionally multi-potent, promiscuous forms (i.e., stem protein types) as a first step towards the divergence of novel effector specificities. Such an evolutionary frame provides a rationale for producing regulators á la carte responsive to synthetic chemical species read more ...